An old star learns new tricks

The pulse of a white dwarf has astrophysicists questioning what we thought we knew about the life of stars, writes Alan Duffy.

The strange object AR Scorpii. In this unique double star a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light.

M. Garlick/University of Warwick, ESA/Hubble

When textbooks are proven wrong, we scientists can’t help but celebrate. So let’s raise a glass to the white dwarf!

We have always dismissed these aged fellows as defunct relics of a sun-sized star. Now one has surprised us. Instead of going off gently into that good night, it is zapping the universe with a spinning beam of radiation. For astrophysicists like me, this is like hearing a retired centenarian has entered the world heavyweight boxing championships and is punching with the best of them.

This unexpected behavior was reported in a January issue of Nature Astronomy by David Buckley at the South African Astronomical Observatory and colleagues from the University of Warwick.

The white dwarf, AR Scorpii, and a larger companion star (a red dwarf) are located 380 light years away. Separated from each other by just three times the distance between the Earth and Moon, they orbit each other every four hours.

Till now, if you had have asked me to describe the typical life story of a white dwarf, my explanation would have gone something like this.

Fast-forward the next five billion years to see the Sun age before your very eyes. Its surface reddens and bloats as fusion reactions relocate to the outer layers; its shapely edges blur as its atmosphere drifts off into space. Now known as a red giant, it engulfs Mercury and Venus, almost certainly Earth and possibly Mars.

At the end of those five billion years, the Sun’s nuclear fusion furnace has used up its fuel. Absent the outward pressure, it collapses under its own gravity.

The result is an Earth-sized object – about one millionth its original size. After 10 billion years of fusion, the Sun is gone, the remaining carbon atoms crushed till they form a near-perfect lattice akin to a diamond. Each teaspoon’s worth of material equals a ton in mass.

It is now a white dwarf. Though the star’s surface continues to glow white hot at more than 100,000 Kelvin, it is effectively dead, slowly fading to leave a black dwarf, with no more role to play in the evolution of the galaxy.

AR Scorpii, however, is different. Rather than fading away, it has been acting more like a lighthouse, spinning on its axis every two minutes and emitting a tightly focused beam of radiation along its magnetic poles. Like a giant dynamo, the beam is powered by a magnetic field a 100 million times that of Earth’s.

In emitting its regular rotating beam, AR Scorpii is behaving like a pulsar, albeit a slow one. These cosmic beacons usually spin with a period of seconds rather than minutes and were previously thought to be powered only by neutron stars, the end state of a star with a mass at least three times that of the Sun. Even more dense than a white dwarf, a teaspoonful of neutron star weighs a billion tonnes.

Even more unusually, the beams from the feisty AR Scorpii tear across the face of its companion star, accelerating material to close to the speed of light and causing it to shine measurably brighter.

Just how AR Scorpii acquired the superpowers of a neutron star is a mystery that has astrophysicists bemused. White dwarves are not supposed to be able to do this! Only neutron stars were thought to be able to power the pulsars seen in their thousands across the galaxy. Now we know different.

This isn’t the first time researchers have suggested a white dwarf might not just be a silent senior citizen. In 2008, Japanese astrophysicist Yukikatsu Terada and colleagues published an article in the Publications of the Astronomical Society of Japan that showed the rapidly rotating white dwarf AE Aquarii was pulsating X-rays.

Just how AR Scorpii acquired the superpowers of a neutron star is a mystery to astrophysicists. White dwarves are not supposed to be able to do this! Only neutron stars were thought to be able to power pulsars. Now we know different.

To change a textbook, it is great to have more than one exception to the rule. Which white dwarf ultimately lays claim to the crown of “first” pulsar heavyweight is less important than the fact that something as extraordinary as an Earth-sized diamond crystal can hold even more surprises for astronomers.